Micro-electrical-mechanical system (MEMS) devices come in a variety of types and are utilized across a broad range of applications. One type of MEMS device that may be used in applications such as radio frequency (RF) circuitry is a MEMS vibrating device (also known as a resonator). A MEMS resonator generally includes a vibrating body in which a piezoelectric layer is in contact with one or more conductive layers. Piezoelectric materials acquire a charge when compressed, twisted, or distorted. This property provides a transducer effect between electrical and mechanical oscillations or vibrations. In a MEMS resonator, an acoustic wave may be excited in a piezoelectric layer in the presence of an alternating electric signal, or propagation of an elastic wave in a piezoelectric material may lead to generation of an electrical signal. Changes in the electrical characteristics of the piezoelectric layer may be utilized by circuitry connected to a MEMS resonator device to perform one or more functions.
Guided wave resonators include MEMS resonator devices in which an acoustic wave is confined in part of a structure; such as in the piezoelectric layer. Confinement may be provided on at least one surface, such as by reflection at a solid/air interface, or by way of an acoustic mirror (e.g., a stack of layers referred to as a Bragg mirror) capable of reflecting acoustic waves. Such confinement may significantly reduce or avoid dissipation of acoustic radiation in a substrate or other carrier structure.
Various types of MEMS resonator devices are known, including devices incorporating interdigital transducer (IDT) electrodes and periodically poled transducers (PPTs) for lateral excitation. Examples of such devices are disclosed in U.S. Pat. Nos. 7,586,239 and 7,898,158 assigned to RF Micro Devices (Greensboro, N.C., USA), wherein the contents of the foregoing patents are hereby incorporated by reference herein. Devices of these types are structurally similar to film bulk acoustic resonator (FBAR) devices, in that they each embody a suspended piezoelectric membrane.
MEMS resonator filter arrays have been studied as a smaller and more highly integrated replacement for the numerous filter dies that currently reside in the radio frequency front-end of a multi-band cellular handset. One method for fabricating multi-frequency MEMS resonator devices including thin plates of lithium niobate is proposed by R. H. Olsson III, et al., “Lamb Wave Micromechanical Resonators Formed in Thin Plates of Lithium Niobate,” Solid-State Sensors, Actuators and Microsystems Workshop, Jun. 8-12, 2014, Hilton Head Island, S.C. (hereinafter, “Olsson” Olsson discloses photolithographic patterning and selective helium ion irradiation (to effectuate helium ion implantation) of a lithium niobate wafer to create a damaged internal release layer, followed by selective etching of the release layer with hydrofluoric acid to form suspended membranes that are overlaid with chromium electrodes. Olsson describes advantages of this process as: (1) the ability to lithographically define the undercut of the device, (2) lack of need for any wafer bonding, polishing, or fracturing, and (3) the ability to realize multiple lithium niobate thicknesses on a single substrate. One limitation associated with processes disclosed by Olsson is that they are not well-suited to create relatively wide areas of uniform thicknesses, owing to the Gaussian profile of an ion implantation beam. Another limitation is that narrow damaged areas may be difficult to release. Yet another limitation associated with processes disclosed by Olsson is their inability to provide electrodes arranged below a suspended membrane. Further limitations with processes according to Olsson are that they require costly ion implantation equipment, and the use of ion implantation has the potential for affecting properties of piezoelectric materials.
Plate wave (also known as lamb wave) resonator devices are also known, such as described in U.S. Patent Application Publication No. 2010-0327995 A1 to Reinhardt et al. (“Reinhardt”). Compared to surface acoustic wave (SAW) devices, plate wave resonators may be fabricated atop silicon or other substrates and may be more easily integrated into radio frequency circuits, Reinhardt discloses a multi-frequency plate wave type resonator device including a silicon substrate, a stack of deposited layers (e.g., SiOC, SiN, SiO2, and Mo) constituting a Bragg mirror, a deposited AlN piezoelectric layer, and a SiN passivation layer. According to Reinhardt, at least one resonator includes a differentiation layer arranged to modify the coupling coefficient of the resonator so as to have a determined useful bandwidth. One limitation of Reinhardt's teaching is that deposition of AlN piezoelectric material (e.g., via epitaxy) over an underlying material having a very different lattice structure generally precludes formation of single crystal material; instead, lower quality material with some deviation from perfect orientation is typically produced. A further limitation is that Reinhardt's approach does not appear to be capable of producing resonators of widely different (e.g., octave difference) frequencies on a single substrate.
Accordingly, there is a need for multi-frequency guided wave devices that can be efficiently manufactured, and that enable production of widely different frequencies on a single substrate. Desirable devices would incorporate high quality piezoelectric materials.